当fluid.layers.pool2d的输入的后两个维度不确定(-1)时，fluid.layers.pool2d报错，求问怎么解决？

EnforceNotMet: Due to the settings of padding(0), filter_size(2) and stride(2), the output size is less than 0, please check again. Input_size:-1 at [/paddle/paddle/fluid/operators/pool_op.cc:40]